Using data from the United States and the United Kingdom, we propose to investigate how family structure and instability create disparities in children's health, both independently and in combination with socioeconomic status and race/ethnicity. Research on inequality in children's health has disproportionately relied on race/ethnicity and socioeconomic status as markers of children's environments, ignoring more proximate determinants of early-life health disparities. In particular, we consider family structure as: 1) an independent contributor to health disparities;2) as a factor that explains some of the observed racial/ethnic and socioeconomic health inequalities;and 3) as a factor that works together in complex ways with race/ethnicity and socioeconomic status to create disparities. Family structure, along with changes in family structure, is expected to independently influence children's health through the quantity and quality of time that parents spend with their children, as well as the quality of children's physical environments within and outside of the home. Differences in family structure may also account for some of the persistent socioeconomic and racial/ethnic disparities that have been observed in previous research. Similarly, observed relationships between income and children's health may be due indirectly to changes in family structure which have led to a decrease in the financial resources available to mothers. Finally, it is unlikely that family structure, race/ethnicity and socioeconomic status work entirely independently of one another to influence children's health. Examining these questions across the U.S. and the U.K. will be particularly instructive, given the similarities in the socioeconomic profiles and traditions of the two countries, combined with differences in their population composition and social policies, especially health care provision. Our analyses are based on data from two nationally representative birth cohort studies that follow children from birth through middle childhood: the Fragile Families and Child Wellbeing Study (FFS), and the Millennium Cohort Study (MCS). Drawing on techniques for cross-sectional and longitudinal analysis, and using several methods for addressing endogeneity and selection, we study the role of family environments in the emergence of health disparities during infancy, early and middle childhood, as well as variation within families and children over time. We also study how the relationship between family structure and children's health differs across policy environments both within and across nations. PUBLIC HEALTH RELEVANCE: Although an abundance of research demonstrates a strong relationship between social status and health in the United States and the United Kingdom, and a growing body of evidence suggests that health disparities begin early in life, analyses that rely solely on race/ethnicity and economic status to understand children's health disparities cannot identify the more proximate determinants of health inequalities. Many of these determinants, which are ultimately of interest to policymakers, are related to family structure. We use data with detailed information on family structure and children's health, as well as substantial socioeconomic and racial/ethnic diversity, to move beyond an elementary understanding of the role of the family in generating inequalities in children's health.